Next generation wireless communication technology includes highly integrated transceiver-antenna tandems, which may be referred to as active antenna systems (AAS's). In active antenna systems, the antenna is actually a phased array integrated into the transceiver. Accordingly, there is no RF connector that can be used for testing the transceiver and/or the antenna separately, as in conventional radio systems. Regardless, test system manufacturers and installers want to know the metrics that were traditionally measured in radio systems having RF connectors to accommodate such measurements. This may be accomplished using “over the air” (OTA) testing. The test metrics include several measurements, including measurements of effective isotropic radiated power (EIRP), total radiated power (TRP), effective isotropic sensitivity (EIS), signal to noise and interference ratio (SNIR), and error vector magnitude (EVM), for example. Although methods are being developed to test EIRP, TRP, EIS, and some other test metrics, there is no method presently available for performing OTA testing of EVM in a compact environment.
EVM OTA testing presents new challenges. For example, to ensure that errors in the modulation format constellation arise from an imperfectly transmitting device under test (DUT), and not from imperfect receive measurements, the measuring equipment necessarily must have high SNR during the test. This may be accomplished in conventional measurements of EVM using RF connectors. For OTA testing, the testing provides signals that are received after wireless propagation, and hence the test equipment must emulate a far field scenario. However, if the measurement is truly made in the far field, the signal is greatly diminished, so the SNR is poor, raising the question of where did the constellation error arise (if there is constellation error). Of course, the same issue is present when the DUT is receiving and the test equipment is transmitting. Thus, there is a need for a compact method of measuring and/or characterizing EVM over the air (OTA) for active antenna systems.